1. Field of the Invention
The present invention relates to an encoder that is usable for various apparatuses such as optical apparatuses and outputs, with relative movement of a scale and a sensor, a signal showing position of a movable member provided in the apparatus.
2. Description of the Related Art
Encoders that are used to detect position and speed of a movable member are constituted by a scale provided with a periodic pattern and a sensor outputting a periodic signal that periodically changes corresponding to the periodic pattern with relative movement of the sensor and the scale. Such encoders include an incremental encoder that, for example, produces two periodic signals (two phase signals) having mutually different phases from one periodic pattern and detects a relative position of the scale and the sensor from number of periods and the phases of the two phase signals. Moreover, the encoders also include an absolute encoder that, for example, produces two paired two phase signals from two periodic patterns having mutually different periods and detects an absolute position of the scale or the sensor from a difference (phase difference) between the phases of one of the two paired two phase signals and the phases of the other paired two phase signals.
Such an absolute encoder has an advantage that a reset operation to detect a reference position for position detection before start of the position detection such as at a time of power-on, which is needed for the incremental encoder, is not needed.
Furthermore, the encoders include one employing a vernier method as an angle detection method as disclosed in Japanese Patent Laid-Open No. 2011-133286. Such a vernier encoder uses a scale provided with multiple tracks including periodic patterns whose periods are mutually different. This vernier encoder calculates a phase difference between each two of multiple periodic signals detected from the respective tracks and having mutually different periods to produce a periodic signal called a vernier signal having a period different from those of the original multiple periodic signals.
In such a vernier encoder that reads the periodic patterns provided at multiple positions in the scale to perform phase difference calculation, a deformation of the scale or an installation error thereof to a member for holding the scale changes relative phases of the multiple periodic signals and thereby causes an error in the phase difference between the periodic signals, which makes it impossible to perform position detection with high accuracy.